Two similar membrane bioreactors of 2 m3 each were operated in parallel over two years under the same operational conditions, fed with the same municipal wastewater. The only process and operational difference between both pilot plants was the position of the denitrification zone (pre-denitrification in pilot 1 and post-denitrification in pilot 2). Despite parallel operation, the two MBRs exhibited different fouling rates and decreases in permeability. These differences could not be accounted for by MLSS concentrations, loading rates, or filtration flux. In a one-year investigation, soluble and colloidal organic material in the activated sludge of both MBR was regularly analysed by spectrophotometric and Size Exclusion Chromatography (SEC) methods. The larger organic molecules present in the sludge water phase (i.e. polysaccharides, proteins and organic colloids) originating from microbial activity (extracellular polymeric substances) were found to impact on the fouling and to explain the difference in membrane performance between the two MBR units. In both pilot plants, a linear relationship could be clearly demonstrated between the fouling rate of the membrane and the concentration of polysaccharides in the sludge water phase during a 5 month operational period at an SRT of 8 days.
Enhanced biological phosphorus removal (Bio-P) in a membrane bioreactor (MBR) promises several advantages but was never attempted as not compatible with high sludge ages. This article includes description and results of bench-scale investigations on Bio-P removal in an MBR. An MBR bench-scale plant (210 L) was operated in parallel to a conventional WWTP under comparable process conditions. The results show that Bio-P removal is possible in MBR. The effluent qualities of the plants were comparable. The effluent P-concentration was always lower than 0.2 mg P(T)/L. In the MBR bench-scale plant P-uptake occurred mainly in the anoxic zone. Investigations with P-spiking showed higher Bio-P potential as P-removal increased up to 20-25 mg/L while P/TS rose up to >6%.
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